1. Field of the Invention
The present invention relates to a CDMA communication system, and in particular, to a power control device and method for controlling a reverse link common channel in a CMDA communication system.
2. Description of the Related Art
Code division multiple access (CDMA) mobile communication systems are based on the IS-95 standard which mainly supports voice service. It is foreseen that mobile communications will be performed in accordance with the IMT-2000 (International Mobile Telecommunication-2000) standard in the near future. The IMT 2000 standard provides not only voice service but also high speed packet service. For example, the IMT-2000 standard supports high quality voice service, moving picture service, Internet search service, etc.
The CDMA mobile communication system includes a forward link for transmitting a signal from a base station (BS) to a mobile station (MS) and a reverse link for transmitting a signal from the mobile station to the base station. A conventional CDMA mobile communication system cannot control power of a common channel for the reverse link. This is because an existing CDMA base station does not have structure for controlling the power of the reverse link common channel. Furthermore, it is difficult to send corresponding power control commands to the respective mobile stations using a forward link common channel in which several users receive one Walsh orthogonal channel. Therefore, it takes a long time for the mobile station to access the system through the reverse link common channel, and the mobile station can transmit a short message only. In addition, the mobile station accesses the system without knowing an appropriate initial system access power, thereby exerting influence on the system.
FIG. 1 illustrates a channel transmission device for transmitting a power control command in a conventional CDMA communication system. The illustrated channel transmission device may be used for a traffic channel or a control channel. In describing the channel transmission device, the input data is assumed to be full rate data of a 20 ms frame.
A cyclic redundancy check (CRC) generator 111 generates 12 CRC bits and adds the generated CRC bits to 172-bit frame data input. A tail bit generator 113 generates 8 tail bits and adds the generated tail bits to an end of the CRC-added frame data to enable an encoder 115 to initialize the data by the frame unit. When the 172-bit data is input, the data output from the tail bit generator 113 becomes 192-bit data. The encoder 115 then generates 576 symbols per frame by encoding one-frame data output from the tail bit generator 113, and an interleaver 117 interleaves the encoded data output from the encoder 115.
A bit selector 121 decimates a long code output from a long code generator 119 to match the length of the long code to the length of the interleaved encoded data. An XOR gate 123 XORs the interleaved encoded data and the decimated long code to scramble them. After that, a signal convert 125 maps the output signal levels of the XOR gate 123 by converting a signal level “0” to “+1” and a signal level “1” to “−1”, and demultiplexes the converted signals by outputting odd-numbered data to an In-phase channel (first channel) and even-numbered data to a Quadrature-phase channel second channel). The I- and Q-channel converted signals are gain controlled in channel gain controllers 127 and 129, respectively.
A control bit gain controller 131 controls a gain of an input power control (PC) bit and provides the gain controlled power control bit to puncturers 133 and 135. The puncturers 133 and 135 puncture symbols located at the bit positions designated by a bit selector 121 and insert therein the power control bits output from the control bit gain controller 131. The symbols output from the puncturers 133 and 135 are multiplied by an Walsh code in multipliers 139 and 141, respectively, thus being orthogonally modulated.
Since, the number of the available orthogonal codes is limited in the CDMA communication system and many traffic channels should be assigned to the users for the data communication service, it is expected that the orthogonal codes will run short. Therefore, when the data communication is temporarily discontinued in the state where the traffic channel is formed, it is preferable to temporarily release an orthogonal code for the channel presently in service and reassign the orthogonal code at the time when the data communication is restarted. This increases utility efficiency in the use of the orthogonal codes.
However, the channel transmission device of FIG. 1 assigns the orthogonal code to transmit the power control command, even when there is no actual data to transmit (in other words, even in the case where data transmission is temporarily discontinued), thus resulting in an inefficient use of the orthogonal codes.